Various substrate materials, such as silicon, silicon carbide, and sapphire have been used in the manufacture of semiconductor products including LEDs, laser diodes, and photo detectors. With the advent of newer semiconductor technologies, such as those used to produce blue LEDs using type III–V nitride compounds, these traditional substrate materials have turned out to have certain handicaps.
For example, growing a gallium nitride (GaN) semiconductor layer on a silicon substrate has proven to be difficult for several reasons, one of which relates to the differences in linear expansion coefficients between silicon (Si) and GaN. While Si has a linear expansion coefficient of ˜2.6×10−6/K, GaN has a linear expansion coefficient of ˜5.59×10−6/K. Given this difference in expansion coefficients, the application of heat during the growing process causes the Si substrate to expand disproportionately with reference to the expansion of the GaN layer thereby leading to misalignment of the lattice structures between the two materials, or in a more drastic failure situation leading to fractures and cracking of the GaN layer.
Similar to Si, the use of sapphire as a substrate also suffers from several handicaps. One handicap is the cost of the material, while another is the occurrence of crystal lattice mismatch between the sapphire substrate and the deposited GaN thin film, during the manufacture of devices such as blue LEDs. Drawing attention to FIG. 1, it can be observed that a mismatch exists between the unit cell lattice 105 of GaN and the unit cell lattice 110 of sapphire. This mismatch is typically of the order of ˜33%. The nitride growth associated with growing GaN upon a sapphire substrate normally occurs on this oxygen sub-lattice 115, which has a slightly distorted hexagonal structure. The mismatch between the unit cell lattice 105 of GaN and the oxygen sub-lattice 115 of sapphire is typically of the order of ˜16%. In achieving this value, the GaN unit cell has to be rotated about the C-axis by 30 degrees, resulting in a slightly distorted GaN unit cell. This lattice-mismatch problem may be resolved in a sub-optimal manner, by introducing a buffer layer between the sapphire substrate and the GaN film. Such a buffer layer creates manufacturability issues leading to low yield and high production costs besides reducing the performance of many devices. As an alternative to sapphire, silicon carbide (SiC) has also been used as a substrate for growing GaN thin films. Similar to the sapphire-GaN crystal structure mismatch, the SiC—GaN crystal structure is also mismatched significantly. Also, as per Chai, “in addition to the poor lattice matching, SiC has three additional problems: growth, defects and fabrication. SiC single crystal is produced by physical vapor deposition method at very high temperatures (>2300° C.). The equipment is expensive and the growth process is slow. Moreover, current technology is limited to 40 mm in diameter and the maximum boule length is approximately 50 mm. Secondly, since the growth is invisible, it is not easy to control the growth process and the crystal defects can be very high, including inclusions and hallowed pipe defects. At present, there is no good solution to improve the growth and to eliminate these defects. Thirdly, SiC is a very hard material approaching to the hardness of diamond and it has been used extensively as abrasives. Therefore, wafer slicing and subsequent polishing are very slow processes. In addition, the combination of these problems further adds to the cost of these substrates. Based on these reasons, SiC is not a good substrate for III–V nitride compound semiconductor thin film growth.”
To achieve good quality epitaxial thin film growth, it is necessary to have the substrate lattice matched as closely as possible, preferably better than 0.01%, to the lattice of the film material. The lattice match limitations imposed by the use of traditional substrate materials such as sapphire and SiC for growing III-nitride films, highlights the need to identify alternative substrate materials that can provide acceptable lattice matching characteristics, while also providing additional advantages such as strong non-linear behavior, and good ferroelectric, pyroelectric, piezoelectric, acoustic, and optical properties. These properties are very useful in the manufacture of devices such as optical waveguides, wavelength converters, and bulk-acoustic wave devices, which can optionally integrate acoustic/electronic/optical/ferroelectric/pyroelectric/piezoelectric circuits together on a common substrate. For reasons of cost, yield and reliability, it is desirable that such integration be carried out using heterogeneous epitaxy fabrication techniques.
It is also strongly desirable to identify substrates that can play an active role in the fabrication and/or operation of semiconductor devices, unlike traditional substrates that are often limited to operating as a foundation for epitaxy, and at best only contribute to device performance via electrical conduction or electrical insulation. An example of such a desirable active role includes the fabrication of an epitaxial film whose spontaneous polarity can be selectively set in conjunction with the ferroelectric polarity of a suitable substrate.